For what it’s worth, the plus sign is probably more universally understood than the concept of shopping cart / basket / bag. It does not necessarily mean ‘Add X to Y’, however, but ‘Increase Amount of X’ – by 1 if not indicated otherwise – which may include automatic addition of another X to Y, especially if there is already at least 1 X in Y. Where amounts can be increased, they usually can also be decreased, so there would be a minus sign, too. That’s all true for English as well, of course.
Drag-drop is quite universal, by the way, but hardly supported by e-commerce sites as far as I know. It quite accurately resembles the physical action of putting some item into a container. This action can also be visualized with a down arrow and a generic container. Many online shopping sites just use a symbolic image of their metaphor for an container (without an arrow or plus sign) instead, i.e. most often a shopping cart, which can look quite different around the world. Note that in the physical world, most people will use a cart (or basket) for grocery shopping, but not for clothing for instance.